Method and apparatus for hydrogen generation

ABSTRACT

A method for configuring a solar hydrogen generation system and the system optimization are disclosed. The system utilizes photovoltaic modules and an electrolyte solution to efficiently split water into hydrogen and oxygen. The efficiency of solar powered electrolysis of water is optimized by matching the most efficient voltage generated by photovoltaic cells to the most efficient input voltage required by the electrolysis cell(s). Optimizing PV-electrolysis systems makes solar powered hydrogen generation cheaper and more practical for use as an environmentally clean alternative fuel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/545,379, filed on Feb. 18, 2004. The disclosure of the aboveapplication is incorporated herein by reference.

FIELD OF THE INVENTION

This system relates to a method and apparatus for electrolysis of waterand, more particularly, to photoelectrochemical (PEC) devices andphotoelectrolysis of water to produce hydrogen.

BACKGROUND OF THE INVENTION

Currently the major process by which hydrogen is produced is by thesteam reforming of methane. Another means of making hydrogen is by theelectrolysis of water. The electricity required for electrolysis ismainly derived from the electric power grid, and the predominant sourceof grid electricity, combustion of fossil fuels, generates emissions,such as nitrogen oxides and particulate matter, as well as carbondioxide. One way to eliminate such emissions is to use solar generatedelectricity to electrolyze water to make hydrogen. Presently, effortsare directed toward improving the efficiency, durability, and cost ofthe solar powered hydrogen production processes.

However, systems consisting of solar cells to make electricity togetherwith electrolyzers to dissociate water into hydrogen and oxygen, as theyexist today, cannot produce hydrogen as cheaply as the steam reformingof methane. Several projects have attempted to produce hydrogen gas tosupply vehicle-fueling stations by using electricity from photovoltaicpanels and commercially available electrolyzers to split water. Theseprojects proved unsatisfactory and were usually short-lived due to thelow efficiency and high cost of the combined technology, which onlyconverted about 2%-6% of the solar energy to hydrogen fuel energy, thus,greatly increasing capital costs, the resulting hydrogen fuel cost (atleast $11 per kilogram of hydrogen), and the large area covered by thesystem.

SUMMARY OF THE INVENTION

To overcome the disadvantages of the prior art, a method for designingand operating a solar hydrogen generator or solar powered electrolysissystem having at least one photovoltaic (PV) module is provided. Thesystem and method utilize at least one PV cell, supplying power toelectrolyze water to produce hydrogen. The method uses the steps ofdetermining the most desirable maximum power point voltage (E_(mpp)) forthe PV module based on a predetermined relationship between actualoperating voltage and actual operating current under load for the PVmodule and the most efficient operating voltage for the electrolysissystem, electrolysis cell or electrolyzer unit (the electrodes andelectrolyte used to split water). Next, the number of solar cells inseries operating at the E_(mpp) needed to achieve the most desirableE_(mpp) of the entire PV module is determined. The most desirableE_(mpp) is the desired voltage needed to split water into hydrogen andoxygen and satisfy voltage losses (the over-voltage and resistances)that is required to operate the electrolysis system and achieve themaximum efficiency for converting solar energy to hydrogen fuel energy.The terms PV cell and solar cell are used in the art and hereininterchangeably. The term PV module refers to one or more cells. Theterm cluster is used interchangeably with the term module.

In another embodiment of the invention, a method for operating a solarpowered electrolysis system having at least one photovoltaic (PV) moduleis disclosed. The system is made up of one or multiple individual solarcells connected in series, supplying power to an electrolysis system toelectrolyze water to produce hydrogen. The method determines the mostefficient actual operating voltage and actual operating current of theelectrolysis system and matches them (making them as closely equal aspossible) to the maximum power voltage E_(mpp) and maximum power pointcurrent I_(mpp) drawn from the PV module to operate the electrolysisprocess. In one aspect, the operating voltage (E_(oper)) of the solarpowered electrolysis system is determined by testing or other means andshould be matched as closely as possible to the maximum power pointvoltage of the PV module (E_(mpp)) based on a predetermined relationshipbetween E_(mpp) for each individual solar cell and E_(mpp) for a PVmodule constructed from one or several solar cells in series. The numberof cells in series (at their maximum power point under the load of theelectrolysis system) needed to achieve the most efficient voltage tosplit water into hydrogen and oxygen and satisfy electrolysis systemlosses (over-voltage and resistances) is then determined.

In another embodiment, a method for operating a photoelectrolysis systemhaving at least one photovoltaic (PV) solar cell or module of multiplesolar cells connected in series or parallel circuits to an electrolysissystem for supplying power to electrolyze water to produce hydrogen isprovided. The number of PV cells in series to achieve a desired voltageto split water into hydrogen and oxygen and satisfy electrolysis systemlosses and resistance (the over-voltage) based on the maximum powerpoint voltage of the PV system (module) is determined, based on apredetermined relationship between photoelectrolysis efficiency andoperating voltage of the electrolysis system.

Another embodiment discloses a method for operating an electrolysissystem having at least one photovoltaic (PV) cell, with two electrodes(the anode and cathode) both connected to the PV system to electrolyzewater to produce hydrogen. Electrically conductive electrodes ofcatalytic materials or coating the surface of the electrodes withcatalytic coatings to split water into hydrogen and oxygen and reduceelectrolysis system losses and resistances (over-voltages) are sizedbased on the current and maximum power point voltage (E_(mpp)) of the PVand electrolysis systems. A maximum current density of the electrodesrequired for efficient operation based on a predetermined relationshipbetween electrolysis efficiency and current density is determined. Byconvention, in electrolysis, the cathode is the electrode wherereduction takes place and hydrogen is generated, and it is attached tothe negative pole of the solar cell. The anode is the electrode whereoxidation takes place and oxygen is generated, and it is attached to thepositive pole of the solar cell.

In another embodiment, a method for operating a solar hydrogen generatoror photoelectrolysis system is disclosed. Each solar PV module of thesystem has at least one solar cell and produces the optimum voltage foroperating the electrolysis system to produce hydrogen. The PV module isconnected to its own separate electrolysis circuit (consisting of ananode, cathode, and electrolyte solution) rather than connecting severalPV modules in parallel to a single electrolysis circuit.

In another embodiment, a solar hydrogen generator or photoelectrolysissystem having at least one photovoltaic (PV) module delivering 1.8 to2.5 volts DC (E_(oper)) is disclosed. The PV module has a currentdensity as low as possible, which must be less than 12 milliamps percm², a nickel-based cathode, and an anode comprising a ruthenium oxidelayer or coating on a nickel-based electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a prior art photovoltaic-electrolyzer hydrogen generationsystem made from a prior art photovoltaic power supply, electrolyzer,and other hardware;

FIG. 2 is a simplified and optimized design for aphotovoltaic-electrolysis (PV electrolysis) system using solar cellswith the optimum electrolysis potential connected in parallel to analkaline electrolysis cell with low over potential electrodes—Ni cathodeand Ni—RuO₂ anode according to the teachings of the present invention;

FIG. 3 shows measurement methods for PV-electrolysis parameters;

FIG. 4 is a graph depicting the effect of different anode materials onthe PV-electrolysis efficiency (solar energy to hydrogen energyconversion efficiency);

FIG. 5 is a diagram depicting the effect of electrode current density onthe electrolysis efficiency;

FIG. 6 is a diagram depicting the open circuit voltage (E_(oc)) ofvarious numbers of crystalline silicon (c-Si) solar cells in series usedto test the PV-electrolysis efficiency;

FIG. 7 is a diagram depicting electrolysis operating current (I_(oper))measured in the PV-electrolysis tests with various numbers of c-Si solarcells in series;

FIG. 8 is a diagram depicting the efficiency of PV-electrolysis systems(the solar energy to hydrogen conversion efficiency) as a function ofthe number of c-Si solar cells in series;

FIG. 9 is a diagram depicting E_(oc) corresponding to the optimumefficiency of PV-electrolysis systems measured with a range ofpotentials from Connecticut Solar c-Si solar cells in series with a highefficiency electrolysis cell;

FIG. 10 is a diagram depicting current and power versus potential for aphotovoltaic module. The current at the maximum power point (mpp) isidentified;

FIG. 11 is a diagram depicting the optimum efficiency range forPV-electrolysis using Connecticut Solar c-Si solar cells wired inseries. This figure also shows several other parameters includingE_(oc), E_(mpp), and I_(oper) as a function of the number of c-Si solarcells wired in series to an alkaline electrolysis cell with low overpotential electrodes—Ni cathode and Ni—RuO₂ anode;

FIG. 12 is a photograph showing a-Si and c-Si solar modules and the tankreactor and electrodes used to optimize the efficiency of hydrogenproduction by PV-electrolysis;

FIG. 13 is a simplified and optimized design for a PV-electrolysissystem using solar cells with the optimum electrolysis potentialconnected in series to an alkaline electrolysis cell with low overpotential electrodes—Ni cathode and Ni—RuO₂ anode. MultiplePV-electrolysis systems as in FIG. 13 are also contemplated by theinvention; and

FIG. 14 is diagram depicting a simplified and optimized design for aPV-electrolysis system as in FIG. 2 using solar cells with the optimumelectrolysis potential connected in parallel to an alkaline electrolysiscell with low over potential electrodes—Ni cathode, Ni—RuO₂ anode; andshowing detail of 6 cells in series per module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The system provides a practical, non-polluting technology for producinghydrogen fuel using photovoltaic semiconductor materials, anelectrolyzer, and sunlight, to power fuel cell vehicles and stationarypower generation at a cost competitive with other energy sources.

The system, which according to the teachings of the present invention,provides a more efficient solar powered PV-electrolysis system forhydrogen generation, was designed by systematically integrating thephotovoltaic circuit and the electrolysis system and optimizing theirefficiencies. The optimization process increased the conversion of solarenergy to hydrogen fuel energy (the system efficiency) from about 2-6%estimated for prior art PV panel and electrolyzer systems to 7.2% for anoptimized integrated system constructed using the same PV materials(crystalline or amorphous silicon). The PV circuit, PV voltage,electrode materials, electrode size, and electrolyte were all integratedand optimized to achieve the highest efficiency.

Costs for PV-electrolysis production of hydrogen fuel were estimatedusing standard cost analysis approaches. The increase in efficiency bythe present system using the same PV material, means that the expensesfor the PV modules needed to generate hydrogen for a fleet of fuel cellvehicles or to generate stationary electric power from non-pollutingrenewable hydrogen would decrease by about a factor of 3.5. The expensesare also reduced because the charge controller, batteries, and DCconverter included in a prior art PV-electrolyzer system are alleliminated by the more efficient and simplified design of the currentsystem. The overall reduction in capital cost due to optimization ofPV-electrolysis is expected to be at least 75%. The estimated cost ofhydrogen fuel generated by PV-electrolysis, which results mainly fromcapital costs, would be reduced from $11 per kg for the prior art system(see FIG. 1) to about $3 per kg. This decrease in production costs fornon-polluting solar hydrogen can be expected to assist in building afuture hydrogen economy to largely replace fossil fuel consumption. Anychange to hydrogen fuel would eliminate the associated green house gasemissions and resulting global warming.

As shown in FIG. 1, the prior art PV-electrolysis systems for solarhydrogen fuel production consisted of an array of photovoltaic modulesmade up of many solar cells connected in series to give a high voltage,a DC-DC converter, voltage controller, storage batteries, and anelectrolyzer, usually consisting of platinum catalyzed electrodes,separated by a specialized membrane, and an aqueous alkalineelectrolyte.

PV modules of an exemplary prior art system consist of 36 crystallinesilicon (c-Si) solar cells connected in series to produce electricitywith an open circuit potential of approximately 22 volts, when notoperating under any load, and a potential of 18 volts at their maximumpower point (E_(mpp)), when delivering their highest current under loadto power processes such as electrolysis or charging batteries. Thesemodules and voltages are usually designed specifically to charge batterysystems and to power electric appliances either directly or through ACinverters and are not optimized to directly or indirectly powerelectrolysis cells. The 18-volt potential of the prior art PV panels istoo high to efficiently generate hydrogen when connected directly toelectrolysis cells (electrolyzer devices).

With general reference to FIG. 2, an efficient electrolysis cell, whichwas used in the present system comprised a platinum (Pt) or nickel (Ni)cathode 30 and a catalyzed, ruthenium dioxide-coated nickel or titanium(Ni—RuO₂ or Ti—RuO₂) anode 32 immersed in 5 M potassium hydroxidesolution 34. Because these electrode materials catalyze the watersplitting reactions, the resulting electrolysis cell 36 has low overpotential and high efficiency. Thus, the electrolysis of water in anefficient electrolysis cell begins at about 1.6 volts DC, which includes1.23 volts, the theoretical potential required to split water, and aminimal overpotential of 0.35 volts for the best catalyzed electrodesystems. Traditionally, the most efficient potential for hydrogenproduction using a PV power source and commonly available electrolysissystems, is usually considerably higher than 1.6 volts due to thelimited catalytic capability of most electrode materials, theresistances in the PV-electrolysis circuit including the electrolyte,and the resulting higher overpotential is about 0.5-1.2 volts. Inpractice, PV devices must usually supply an operating potential(E_(oper)) of about 1.8-2.5 volts measured between the anode and cathodewhen the PV system is connected to a high efficiency alkalineelectrolysis cell.

The over potential represents electricity which does not go for usefulsolar energy conversion to fuel energy (electrolysis) but, instead, isconverted to heat. The lower the E_(oper), at which electrolysis occurs,the more efficient the electrolysis cell, because the over potential andthe energy wasted as heat formation is minimized.

Designing a PV module 38 that is integrated with the PV-electrolysissystem so that it powers the water splitting reaction most efficientlyis a key element in optimizing solar hydrogen production. It isnecessary to optimize both the PV-electrolysis system 40 which powerselectrolysis for hydrogen generation and the electrolysis cell orelectrolyzer 36 so that the PV-electrolysis system voltage at itsmaximum power output (E_(mpp))matches the optimum voltage for theelectrolysis cell and, thus, can produce the highest current and highesthydrogen production efficiency. Thus, the most effective approach foroptimizing solar powered PV-electrolysis for hydrogen productionrequires systematically integrating the photovoltaic circuit and thePV-electrolysis system and optimizing their efficiencies. The PVmaterials, PV circuit, PV voltage, electrode materials, electrode size,and electrolyte type and concentration must all be optimized and thencombined to achieve the highest efficiency. Improving hydrogenproduction efficiency is a major method for reducing the cost ofhydrogen fuel production. Simplifying the PV-electrolysis system byconnecting the PV modules directly to the hydrogen and oxygen electrodesis another means of improving performance and reducing cost, which willbe explained below.

The relationship of hydrogen generation efficiency to PV electricalgeneration efficiency and the efficiency of the electrolysis cell(electrolyte and electrodes) is given by Equation 1.H₂ Generation Efficiency=PV Efficiency×ElectrolysisEfficiency  (Equation 1)

To optimize the generation of hydrogen from a PV-electrolysis system 40,it is necessary to make several improvements in the prior art designshown in FIG. 1: (1) the maximum power point potential (E_(mpp))supplied by the PV-electrolysis system must match the characteristicoperating potential (E_(oper)) required by the PV-electrolysis system,(2) the batteries, which are an inefficient means of energy storage,should be eliminated, and (3)any parts that increase resistance, voltagelosses, and inefficiency in the PV-electrolysis circuit, including thevoltage converter and charge controller should be eliminated as well.The circuit, according to the teachings of the present invention, withthe unwanted parts removed, is shown in FIG. 2. As shown in the examplein FIG. 2, the circuit connections of the solar cells 42 of the PV panel44 have been redesigned by using solar modules with the best number ofsolar cells in series that give the correct E_(mpp) of approximately 2.5volts DC (ranging about 2 to 3 volts DC for catalyzed electrodes) tomatch the operating potential (E_(oper)) required by the PV-electrolysissystem 40. As shown in FIG. 2, three (or more) of the 2.5 volt modulescan be connected in parallel to the electrolysis cell to give a greatercurrent and rate of hydrogen production. Any type of solar cells couldbe used in the optimization process, including but not limited to c-Si,a-Si, CulnSe₂ or, CdTe based solar cells, as long as the number andconfiguration of the solar cells gives the correct E_(mpp) to match thebest operating voltage for the PV-electrolysis system.

Because the 2.5 volt PV modules, shown connected in parallel in FIG. 2,must have equal E_(mpp) and I_(oper) (a quality called voltage andcurrent matching) for optimal performance and efficiency, a problem canarise if one or more of the modules is partially shaded from thesunlight during part of its operation or becomes defective due to age ordamage (or for other reasons) and no longer makes the same E_(mpp) andI_(oper) as the other modules. If it is judged that this mismatching mayhappen, then each of the modules can be connected to a separateelectrolysis cell (anode and cathode) rather than being connected inparallel to the same electrolysis cell as shown in FIG. 2. Connectingeach optimized PV module to a separate electrolysis circuit in this waycould, thus, increase the efficiency of the entire solar hydrogenproduction system by preventing current mismatching. The increase inefficiency would depend upon the seriousness of the mismatching itavoids. The greatest benefit comes from connecting each individual PVmodule to a completely separate electrolysis cell with a separate anodeand cathode in a separate electrolyte container insulated from any otherelectrolysis cell. At least, part of the maximum benefit could beobtained by connecting each PV module to a separate anode and cathode,but immersing more than one anode and cathode pair in the same containerof electrolyte to save expense, in which case the mismatched PV moduleswould be only partially insulated from each other. See also FIG. 13showing a module having several cells in an electrolysis system.Multiple electrolysis systems replicating that of FIG. 13 may optionallybe used. Also, multiple electrode sets in a single container, eachassociated with a PV module are also contemplated.

The 2.5-volt modules, shown in FIG. 2, are optionally made byredesigning a typical PV panel 44. For example, some PV panels 44 have36 crystalline solar cells in series. Each of these cells might have aninherent open circuit potential (E_(oc)) of 0.6 volt, and the entire PVpanel might have an E_(oc) of 21.6 volts. Each of these solar cellscould have an E_(mpp) of 0.41-0.5 volt, and several cells could beconnected in series to make a solar module with the required optimumE_(mpp) of 2 to 3 volts to power water electrolysis. For example, sixsolar cells (CT Solar, Putnam, Conn.) connected in series can give anE_(mpp) of 2.46 volts (6×0.41=2.46), which matches the E_(oper)requirement of an efficient electrolysis cell (1.8-2.5 volts) describedabove, as will be explained below in the experimental results section.FIG. 3 illustrates the methods used to measure the E_(oc), the shortcircuit current (I_(sc)), the operating potential during electrolysis(E_(oper)) and the operating current during electrolysis (I_(oper)),where the symbol V in a circuit indicates a voltmeter and A indicates anammeter. E_(mpp) of individual solar cells can be estimated from theE_(mpp) of the PV module determined by the manufacturer and the numberof cells in series (Equation 2).

$\begin{matrix}{{E_{mpp}\mspace{14mu}{of}\mspace{14mu}{each}\mspace{14mu}{solar}\mspace{14mu}{cell}} = \frac{E_{mpp}\mspace{14mu}{of}\mspace{14mu}{PV}\mspace{14mu}{module}}{{{No}.\mspace{14mu}{of}}\mspace{14mu}{solar}\mspace{14mu}{cells}\mspace{14mu}{in}\mspace{14mu}{series}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

The E_(mpp) and efficiency of crystalline silicon PV cells are normallymeasured at standard test conditions which are 25° C. and 100 mW/cm²(one sun irradiance). However, E_(mpp) and efficiency decrease withincreasing temperature. This temperature coefficient effect, may be oneof the reasons (along with the other sources of over potential of theelectrolysis system) that the optimum E_(mpp) of the PV modules (2.0-3.0volts DC) for real world PV-electrolysis systems is significantly higherthan the minimum potential for electrolysis (1.6 volts DC) in highefficiency electrolysis cells.

To optimize PV-electrolysis for hydrogen generation, it is necessary todetermine the efficiency of converting solar energy to hydrogen fuelenergy as a function of the electrode and PV characteristics, includingthe electrolysis efficiency (related to electrode over potentials) andthe PV efficiency, potential, current, and resulting power generatedwhen it is connected to the electrolysis cell. Experimentally, theefficiency of hydrogen generation from PV-electrolysis is determined byconnecting the PV power source, irradiated with sunlight at a knownintensity (usually one sun irradiance, which equals 100 mW/cm²), to ahigh efficiency electrolysis cell or electrolyzer and measuring theelectrolysis current I_(oper). Then, the efficiency is calculated usingEquation 3.

$\begin{matrix}{{Efficiency} = {\frac{{I_{oper}({mA})} \times 1.23\mspace{14mu}{volts}}{{PV}\mspace{14mu}{{area}\left( {cm}^{2} \right)} \times {Solar}\mspace{14mu}{{Irradiance}\left( {{mW}/{cm}^{2}} \right)}} \times 100\;\%}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

In optimization experiments for this embodiment, all the PV systems wereconnected to an electrolysis cell containing a Ni—RuO₂ anode and a Nicathode, each with a surface area of 128 cm², immersed in 450 mL ofaqueous 5 M KOH solution. During electrolysis using PV solar cells withE_(mpp) ranging from 1.5 to 16 volts DC (E_(oc) of 1.8 volts to 20volts) connected to these electrodes, the actual operating potentialdifference (E_(oper)) ranged from 1.5 to 3.3 volts.

It was determined experimentally that alkaline conditions are much lesscorrosive than acidic conditions for all the candidate materials used aselectrodes, contacts, connectors, and in PV devices which would come incontact with the electrolyte. Chiefly, the encapsulation and insulationmaterials, including the plastics, Tefzel, Epoxy, and Acrylic, and theelectrode materials, nickel, nickel coated with RuO₂, platinum, andconductive glass coated with fluorine doped tin oxide (SnO₂:F) were notetched or corroded after immersion for more than one month inconcentrated potassium hydroxide (5 M KOH). It was also determined thatmaximum conductivity (approximately 0.55 Siemens/cm at 20° C.) inaqueous KOH solutions occurs at a concentration of 5 M (22-25% byweight). Thus, a 5 M KOH electrolyte solution was chosen foroptimization of PV-electrolysis systems.

As shown in FIG. 4, the efficiency of a range of electrode materials isshown which allows the optimization of the design of the electrolysiscell by minimizing the system overpotential (over-voltage). The type ofanode, where oxygen gas is generated during electrolysis, was varied,because the anode has greater over potential than the cathode and itsoptimization is, therefore, more difficult. It was already known thatnickel is a highly efficient and inexpensive material for the cathode,where hydrogen gas is evolved. Six candidate anodes were tested: nickel(Ni) and platinum (Pt), which are especially resistant to corrosion inalkaline electrolytes and have catalytic qualities for water splitting,Ni—RuO₂ or Ti—RuO₂, nickel or titanium coated with ruthenium oxide, aknown catalyst for oxygen electrodes, and two other relativelyinexpensive and corrosion resistant conductive materials, glass coatedwith fluorine doped tin oxide (SnO₂:F), a corrosion resistanttransparent conductive oxide, and silver epoxy (Ag-epoxy), a metalpolymer composite used as a sealant and adhesive. The most efficientanode material was found to be nickel-ruthenium oxide (probablyNi—RuO₂), which gave the highest conversion rate from solar energy tohydrogen (4.5%). It should be noted that the E_(oc) (3.8 volts) of thePV solar cells used in the anode tests, two amorphous silicon triplejunction solar cells (from Unisolar, Inc., Troy, Mich.) connected inseries, approximately matched the optimum E_(oc) range of 3.0-4.0 voltsDC). Since the efficiency of the solar cells used in the anode tests forconverting sunlight to electrical energy was rated at 6.9%, theefficiency of the electrolysis process even with the best anode materialwas 4.5%/6.9%×100%=65%. This calculation is derived from Equation 1.Thus, the operation of the optimized electrolysis cell under actualreported conditions was reduced from its estimated maximum efficiencylevel of approximately 80% to only 65% despite a relatively good matchbetween the PV electrical output and the requirements of theelectrolysis system.

The optimum area of the electrodes used in the electrolysis cell dependsupon the effect of current density (J_(oper)) on electrolysis efficiency(see FIG. 5). The results of PV-electrolysis efficiency measurements atseveral current densities indicate that efficiency begins to decreasewhen J_(oper) is greater than about 12 mA/cm². There was no decrease inefficiency under electrolysis conditions used in the optimizedPV-electrolysis devices constructed to demonstrate the current system(preferred embodiment), where the surface areas of the electrodes wereapproximately equal to that of the PV panel, which resulted in a currentdensity of 6 mA/cm² or less.

The cathode (hydrogen electrode) with very good corrosion resistance andlifetime greater than 744 hours consisted of 25.4 micrometer thicknickel foil (99.9%, Strem, Inc).

The anode was prepared from the same nickel foil as the cathode and wascoated with RuO₂ by a process using heat treatment at 350° C. Theprocess sticks the ruthenium compound firmly onto the nickel andoxidizes the original RuCl₃ to form RuO₂ by reaction with the hot air inthe oven. Ruthenium chloride, RuCl₃ (FW 207) with a mass of 1.04 g(0.005 moles), was dissolved in 25 mL of concentrated hydrochloric acidand subsequently diluted to 50 mL with deionized water to yield a 0.1 Msolution. This solution was evaporated just to dryness in a fume hood ona hotplate with stirring. The resulting paste was dissolved in 25 mL of2-propanol to yield a 0.02 M solution. Sheets of nickel foil wereprepared and cleaned by first etching them mechanically with very finesand or emory paper, then etching in 20% hydrochloric acid for 30seconds, rinsing in deionized water, and air drying. The sheets werecoated with the 0.2 M ruthenium chloride solution using a fine spray orbrush and sintered in an electric oven at 350 degrees ° C. for 15minutes. The coating was applied to both sides of each electrode in thisway a total of 4 to 5 times, and the resulting electrode was thenannealed in the oven at 350 degrees ° C. for 1 hour. Heating in air inthe oven oxidized the ruthenium chloride to ruthenium dioxide (RuO₂)which produced a stable layer of RuO₂ approximately 2.3 micrometersthick. Then, the coated electrode was cooled, soaked in 5 M KOH solutionat least 1 hour, and rinsed thoroughly with water to remove any excessunreacted RuO₂. The total coating was approximately 0.1 mg RuO₂/cm².This process yields a stable catalyst coating on the electrode whichdoes not come off during its use in alkaline electrolysis. The resultingelectrode did not lose its coating or catalytic activity over extendeduse (>1000 hr) and was less expensive to make and more efficient than asimilar RuO₂ coated titanium electrode.

The other major system which must be improved to optimizePV-electrolysis is the semiconductor PV module itself. Optimizing the PVmodule system requires that the PV voltage delivered under natural solarradiation match the most efficient voltage for operating theelectrolysis cell (already optimized above). To determine the optimumE_(oc) and E_(mpp) required from a PV panel to produce the most hydrogen(the highest PV-electrolysis efficiency), various numbers of crystallinesilicon solar cells (Connecticut Solar) with open circuit voltage E_(oc)of 0.59 volts were connected in series to the optimized electrolysiscell, with a highly efficient Ni—RuO₂ anode and a Ni cathode, to measurethe current through the electrolysis cell (I_(oper)) and calculate thehydrogen generation rate over a range of various numbers of solar cells,E_(oc), and E_(mpp) (see FIGS. 6-9). The electrolysis operating currentand potential were measured in an electrolysis cell with 128 cm²electrodes and 5 M KOH electrolyte as shown schematically in FIG. 3, andthe efficiency of solar energy conversion to hydrogen fuel energy wascalculated using Equation 3. The efficiency results plotted versus thenumber of solar cells in series are presented in FIG. 8. These resultsshowed that the maximum efficiency occurred using six cells in serieswith a module having an E_(mpp) of 2.5 volts and E_(oc) of about 3.5volts (FIG. 9). It is envisioned that other PV solar cells such astriple junction thin layer amorphous silicon (a-Si), thin layer cadmiumtelluride (CdTe), thin layer copper indium diselenide (CulnSe₂),combined a-Si and c-Si photovoltaics, or others could be used in he samemanner as the crystalline silicon (c-Si) cells in the above example.

Table 1 shows results that were obtained for the solar hydrogengeneration systems obtained as described above using inexpensive siliconPV panels.

TABLE 1 Solar Hydrogen Generation Using Amorphous and Crystalline PVMaterials Best Best Electric Gen. Electrolysis Best H₂ Gen. E_(oc)E_(mpp) Efficiency E_(oper) Efficiency Efficiency Source Type (volts)(volts) (%)* (volts) (%) (%) Unisolar a-Si 3.8 2.7 6.9 2.1 65 4.5 CTSolar c-Si 3.5 2.5 10.9 2.0 66 7.2 *Based on the rated power of theoriginal PV panel operating at its maximum power pointFor the a-Si cells, which have an E_(mpp) of ˜1.35 volts (E_(oc) of ˜1.9volts), two a-Si solar cells are connected in series to give anoptimized voltage approximately equal to the optimized voltage from sixc-Si solar cells in series (Table 1).

As shown by Table 1, the c-Si PV material from Connecticut Solar(Putnam, Conn.) gave the best hydrogen generation efficiency (7.2%) whenoptimized to deliver an E_(oc) of 3.5 volts. The triple-junction a-Sisolar cells were obtained from Uni-Pac 10 panels purchased from UnitedSolar Systems Corporation of Troy, Mich. The a-Si solar cells gave alower efficiency than the c-Si cells due to their inherently lowerelectrical generation efficiency. The operation of the electrolysissystem was about equally efficient (65-66%) for both kinds ofsilicon-based solar cells. This result indicates that there was a fairlygood match between the E_(mpp) of the optimized PV system and theE_(oper) of the optimized electrolysis system in each case. A number ofc-Si solar cells from other sources gave similar optimization results(OK Solar, Shell Solar, and Sharp Solar).

The most efficient PV-electrolysis system made with the ConnecticutSolar c-Si cells produced a current of 0.77 A for a 138 cm² module understandard test conditions (one sun of AM 1.5 global sunlight). Thistranslates to a production rate of approximately 1.04 moles of hydrogenper hour for a 1.0 m² panel of optimized solar cells. This generationrate would require a PV array with an area of 36 m² (dimensions of 19.5feet by 19.5 feet) to keep a fuel cell vehicle supplied with enoughhydrogen (3.2 kg/week) for typical driving requirements (assuming that afuel cell vehicle will average 10,000 miles annually with a fuel economyof 60 mpg). Using the system according to the teaching of the presentinvention to construct and optimize a PV-electrolysis system with highefficiency makes it practical to fuel vehicles with renewable hydrogenand resolve all issues of pollution from motor traffic.

Solar powered photovoltaic-electrolysis systems for hydrogen generationcan be optimized using the methods of this system (1) either roughly bymeans of rules of thumb or (2) more exactly by systematically measuringthe electrical and electrochemical characteristics of the PV andelectrolysis devices to find the most efficient type, voltage, andnumber of the solar cells to give the highest conversion rate of solarenergy to hydrogen fuel energy. The rate of water splitting byelectrolysis using the optimized anode and cathode materials (FIG. 4)was first measured as a function of the potential applied to theelectrodes (operating potential, E_(oper)) using a DC power supply. Itwas found that the current in the electrolysis experiments isproportional to the rate of electrolysis, the production of hydrogengas, and hydrogen fuel energy, according to Faraday's Law (Equation 4)where F is Faraday's constant (˜96,500 coulombs/equivalent weight) andincreases with increasing operating potential.

$\begin{matrix}{{{Mass}\mspace{14mu}{of}\mspace{14mu} H_{2}} = \frac{{equivalent}\mspace{14mu}{weight} \times I_{oper} \times {time}}{F}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

When a PV source is used for the direct current, the DC potentialapplied to the electrolysis system is limited by the output of the PVcircuit, and the electrolysis current is limited by the appliedpotential and the series resistance of the electrolysis system as wellas the response of the PV system to operating under the load created bythe electrolysis system. The potential actually applied by a PV systemis not the open circuit potential E_(oc), but the operating potentialE_(oper) (both measured as shown in FIG. 3). E_(oper) is almost alwaysmuch lower than E_(oc) because of the resistance of the load(electrolysis system). For the PV system to most efficiently power theelectrolysis, the PV modules must be capable of giving their bestperformance under the potential and load conditions required by theelectrolysis cell. The best performance conditions of a PV module orsolar cell are called its maximum power point (mpp), defined by thepotential (E_(mpp)) and current (I_(mpp)), at which it produces itsmaximum power (P_(mpp)=E_(mpp)×I_(mpp)). If the E_(mpp) and I_(mpp)correspond closely to the operating conditions, E_(oper) and I_(oper) ofthe electrolysis system, the efficiency of solar energy conversion tohydrogen will be optimized, giving the greatest hydrogen production fora given system and time. The mpp is found by plotting the current (I)versus potential of the PV device while it is irradiated by sunlight ofa known intensity (Emery, 2003). These measurements are usually carriedout under standard test conditions (STC) using 100 mW/cm² i.e., 1000W/m² of sunlight—called one sun, the equivalent of average summertimesunlight for cloud free conditions at noon in the central northernhemisphere at the surface of the earth (AM 1.5 Global).

An example of the power curve of a PV module, which is generated bymultiplying the current times the potential, is shown in FIG. 10. Themaximum power point (mpp), with characteristic E_(mpp) and I_(mpp)values, is a property of each photovoltaic cell or module of severalmultiple cells in series. As shown in FIG. 10, a PV system gives itsgreatest power when operating under a load with an optimal resistance(E_(mpp)/I_(mpp)). If the circuit resistance is extremely low, thecircuit approaches a short circuit condition under which the powerapproaches zero (FIG. 10). The maximum power point potential (E_(mpp)),which is the potential of the PV system giving its greatest power, isless than the open circuit potential (E_(oc)), where R approachesinfinity and I approaches zero. The optimum PV-powered electrolysissystem must have a characteristic maximum power point such that E_(mpp)and I_(mpp) closely match the operating voltage (E_(oper)) and operatingcurrent (I_(oper)) of the electrolysis cell, i.e., their dividend(E_(mpp)/I_(mpp)) matches electrolysis cell operating resistance(R_(oper)). Since the resistance of an electrolysis cell is non-linearand non-ohmic, it could not be measured directly with a simple ohm meter(as one can measure the resistance of a wire), and resistancemeasurements were not used in optimizing the PV-electrolysis system.

However, the PV-electrolysis system can be effectively optimized withthe process of this system by: (1) using optimized anode and cathodematerials, optimal current density on the electrodes, and optimalelectrolyte concentration, (2) designing the PV module to give theoptimal E_(mpp) 2.5 volts DC that matches the most efficient range ofoperating potential (E_(oper)=1.8 to 2.5 volts). Thus, a rule of thumbfor PV-electrolysis optimization is to use a PV module with E_(mpp) of2.0 to 3.0 volts DC to match the best operating potential of theoptimized electrolysis cell E_(oper) of 1.8 to 2.5 volts. The currentand power curves plotted versus potential for this type of PV module areillustrated in FIG. 11. Some PV-electrolysis systems require E_(mpp)values higher than 2.5 volts DC (up to 3.0 volts DC) to reach themaximum efficiency for hydrogen production from a single electrolysiscell. Several examples of crystalline silicon PV materials wereoptimized by the process of this system. Six c-Si solar cells(Connecticut Solar) connected in series gave the highest optimumefficiency (7.2%) for hydrogen production with E_(mpp) of 2.5 volts DC.The reason for the superior performance of the CT Solar cells is due totheir E_(mpp) value of 0.41 volts per solar cell which allowed six cellsin series to give E_(mpp) of 2.46 volts (6×0.41) which closely matchedthe optimum E_(oper) requirement of the electrolysis cell.

In practical terms, too little potential (E_(mpp) and E_(oc)) isprobably worse than too much because the efficiency falls off moresharply below the optimum potential than above the optimum potential(FIGS. 9 and 11). FIG. 11 also shows the optimum range of PV potential(E_(mpp) and E_(oc)) needed to drive electrolysis with the greatestefficiency. Thus, the detailed procedure for making PV-electrolysissystems would be to use enough solar cells in series to give an E_(mpp)as close as practical to 2.5 volts DC, and preferably in the range of2.0-3.0 volts (and/or an E_(oc) in the range of 3.0-4.0 volts),preferably in the middle or higher part of these ranges rather than thelower end. Thus, if the E_(mpp) of the PV module is less than 2.5 volts(the E_(oc) is less than 3.5 volts), one more solar cell should usuallybe added in series with each module. The additional solar cell in eachmodule would increase E_(mpp) by about 0.41 volts and E_(oc) by about0.6 volts. Several of these optimized solar systems with about 2.5 voltsE_(mpp) can be connected in parallel to the electrolysis cell togenerate hydrogen more rapidly by increasing the total current(I_(oper)) as shown in FIG. 2. It would also be possible to optimize aPV-electrolysis system based on a PV system connected to an electrolyzerwith multiple electrolysis cells in series (i.e., a multicellelectrolyzer stack) by using the same method employed for a solar cellconnected to a single electrolysis cell. In the case of the multicellPV-electrolyzer, the process of optimization would be done in the sameway as for a single electrolysis cell except that the optimum PV voltagerequired to operate the multicell electrolyzer with N number of cells inseries would be equal to approximately N times the voltage required fora single electrolysis cell.

The requirements for an optimized PV-electrolysis system determined inthis system and its preferred embodiment are summarized in Table 2. Themain components of the experimental PV-electrolysis systems optimizedare shown in FIG. 12.

TABLE 2 Characteristics for an Optimized PV-Electrolysis System forHydrogen Production Characteristics Benefits Electrolyte 5 M (22%-25% bymass) Maximum electrolyte con- KOH ductivity & electrode durabilityElectrodes Ni cathode & Ni-RuO₂ Optimum electrode effi- anode ciency &minimum over potential Current Less than 12 mA/cm² Electrodes sized fordensity maximum efficiency PV Module Design with E_(mpp) of Optimummatch of PV vol- 2.5 (2.0-3.0) volts DC tage with electrolysis systemvoltage

The features of this optimization process include:

An optimization process for scaling up solar hydrogen production thatyields up to 7.2% efficiency, which is greater than any reportedPV-electrolysis efficiency.

Reducing cost for hydrogen production to about $3 per kg due to thesimplified, 7.2% efficient system and eliminating many prior artcomponents such as voltage converters, controllers, and batteries.

The efficiency optimization process described above which requiressimple efficiency determination using measurements of current and thearea of the PV system and use of manufacturer's mpp specifications toestimate the number of solar cells in series that give maximum hydrogengeneration efficiency.

An optimized PV-electrolysis system design and preferred embodimentconsisting of: (1) a nickel foil cathode (hydrogen productionelectrode), (2) a specially treated ruthenium dioxide (RuO₂) coatednickel foil anode as described above (oxygen production electrode), (3)5 M aqueous potassium hydroxide electrolyte solution, (4) a PV systemconsisting of 6 or 7 crystalline silicon solar cells in series toproduce the optimum E_(mpp) of 2.0 to 3.0 volts, (5) an external circuitsuch as a wire or ammeter as shown in FIG. 2 to electrically connect thePV system to the electrodes in the electrolysis chamber and (6) anelectrolysis chamber consisting of cathode and anode sections separatedby an impermeable divider from the top of the chamber to a point belowthe bottom of the electrodes, which allows the bubbles of hydrogen andoxygen gas to rise to the top of the chamber separately. The electrolytein the anode and cathode sections is connected, however, by an openingor salt bridge at the bottom of the chamber as shown in FIG. 2 toprovide electrochemical connection between the electrodes that isnecessary for ion transport and electrolysis to occur. The balance ofthe gas generation system needed for producing hydrogen fuel requiresonly commercially available gas purification equipment, piping,compression, and storage modules.

The optimized PV-electrolysis system in this system can be givenappropriate corrosion resistant encapsulation to become aphotoelectrochemical (PEC) device in a self-contained, scaled-up solarhydrogen generator using a photoelectrochemical device in a plasticspecially designed chamber to increase the rate of the hydrogenproduction by focusing and concentrating natural sunlight on the PVsystem. This could potentially raise the hydrogen production rate byseveral-fold. Using the optimized design of this system in such specialchamber further reduces the cost of hydrogen fuel productionsignificantly by reducing the area of the PV panels needed to achievethe optimal current density in a PEC reactor.

The cost estimate for renewable solar hydrogen generation for thePV-electrolysis system, according to the present invention, is projectedto be as low as gasoline for the same mileage. Optimized PV-electrolysisproduction of hydrogen could cost $2-3 per gallon of gasolineequivalent, compared to more than $11 per gallon of gasoline equivalentfor solar-generated hydrogen from current photovoltaic and electrolyzertechnology.

The scaled-up, optimized PV-electrolysis design of this system could bebuilt with currently available, commercial manufacturing processes,utilizing commercially available photovoltaic cells and electrolyzers tomake inexpensive, practical fueling systems able to produce hydrogen ona home fueling scale or for various large or small fleet fuelingprojects without air pollution or global warming. The efficiency,durability, and cost of the resulting solar hydrogen systems have beenexplored and would be competitive with conventional fossil fuels. Also,the scale up in the size of this device should be linear, since it wouldmerely involve connecting several PV modules to larger electrolysiscells and/or connecting the hydrogen gas output of many small reactorsto a common storage system.

Current photovoltaic modules connected to prior art electrolyzers couldalso be used to split water, but the cost of the hydrogen (more than $11per gallon of gasoline equivalent) would be much higher than hydrogenfrom our optimized PV-electrolysis design or PEC devices which do thesame thing. Either of the photovoltaic devices mentioned earlier (singleor multi-junction amorphous silicon and crystalline silicon) could beused in the system with a KOH solution and sunlight.

The optimized PV-electrolysis system has an efficiency of about 7% forconditions referred to as AM1.5 global (this is approximately noon timesun on a cloud-free day in the summer in the northern U.S.). Suchconditions can provide the equivalent of approximately 6-8 hours ofirradiance at 1000 W/m². For such conditions, the system provides apractical means of supplying renewable, non-polluting hydrogen fuel nowby partnering with suppliers to make and further improve such systems.The solar hydrogen fueling systems could be built in most parts of theU.S. although they would be most productive in the desert Southwest andsunbelt areas. The system works, albeit at a reduced hydrogen output, oncloudy days. The protective surfaces that are currently used oncommercially available solar cells (glass and plastics resistant toattack by ozone and other atmospheric pollutants) also resist attack by5 M KOH used in our electrolysis system.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A method for operating at an optimized condition an electrolysissystem having at least one photovoltaic (PV) cell, supplying power toelectrolyze water to produce hydrogen, comprising: a) determining amaximum power point voltage (E_(mpp))for said PV cell based on apredetermined relationship between actual voltage and actual currentunder load for said PV cell; and b) determining a number of cells atsaid E_(mpp) to be connected to achieve a desired voltage to electrolyzewater and satisfy required electrolysis system losses.
 2. The method ofclaim 1, wherein said number of cells are in series and constitute amodule.
 3. The method of claim 2 wherein said module has an overallE_(mpp) in a range of 2 to 3 volts.
 4. The method of claim 2 whereinsaid module has an overall E_(mpp) of about 2.5 volts.
 5. The method ofclaim 1 wherein the desired voltage is an operating voltage in a rangeof 1.5 to 3.0 volts.
 6. The method of claim 1 wherein the desiredvoltage is an operating voltage in a range of 1.8 to 2.5 volts for asingle electrolysis cell.
 7. The method of claim 1 wherein said E_(mpp)of said number of cells corresponds to an open circuit voltage in arange of 3 to 4 volts.
 8. The method of claim 2 wherein said E_(mpp) ofeach said cell is about 0.41 volts and said cluster contains six cellsconnected in series.
 9. The method of claim 1 wherein said E_(mpp) ofeach said cell is about 0.41 volts and said desired operating voltage isabout 2.5 volts.
 10. The method of claim 2 wherein a plurality of saidmodules are arranged in parallel.
 11. The method of claim 10, whereinhydrogen production of said system is proportionate to the number ofsaid multiple modules.
 12. A method for operating at an optimizedcondition an electrolysis system having at least one photovoltaic (PV)cell supplying power to electrolyze water to produce hydrogen,comprising: a) determining a relationship between actual voltage andactual current under load for at least one of the PV cells; b)determining a maximum power point voltage (E_(mpp))for said PV cellsbased on said relationship; and b) determining a number of cells inseries at said E_(mpp) to achieve a desired voltage to electrolyze andsplit water and satisfy electrolysis system losses.
 13. A method foroperating an electrolysis system having at least one photovoltaic (PV)cell supplying power to electrolyze water to produce hydrogen,comprising: determining a number of PV cells in series to achieve adesired voltage to electrolyze water and satisfy electrolysis systemlosses based on the maximum power point voltage (E_(mpp))of said PVcell.
 14. A method for operating at an optimized condition anelectrolysis system having at least one photovoltaic (PV) cell, andelectrodes coupled to the PV cell, to electrolyze water to producehydrogen, comprising: determining a number of PV cells to achieve adesired voltage to electrolyze water and satisfy electrolysis systemlosses based on a maximum power point voltage (E_(mpp))of said PV cell;and determining a maximum current density of said electrodes based on apredetermined relationship between electrolysis efficiency and currentdensity.
 15. The method of claim 14, wherein said current density isbased on electrolysis current and PV cell area.
 16. The method of claim14 wherein said PV cell comprises a a-Si-based multi-junctionsemiconductor material or a c-Si based single junction cellsemiconductor material.
 17. The method of claim 14 wherein one of saidelectrodes is one of a photoanode or a photocathode.
 18. The method ofclaim 14 wherein said desired voltage is an operating voltage in a rangeof 1.5 to 3.0 volts.
 19. The method of claim 14 wherein said desiredvoltage is an operating voltage greater than 1.5V.
 20. The method ofclaim 14 wherein said desired voltage is an operating voltage greaterthan 1.8V.
 21. The method of claim 14 wherein said E_(mpp) voltage ofsaid number of cells is greater than or equal to 2.5V.
 22. The method ofclaim 14 wherein said E_(mpp) voltage of said number of cells is in arange of 2 to 3 volts.
 23. The method of claim 14 wherein said operatingvoltage corresponds to an open circuit voltage in a range of 3 to 4 V.24. The method of claim 14 wherein said open circuit voltage is lessthan 5V.
 25. The method of claim 14 wherein said E_(mpp) voltage of saidnumber of cells is less than 4V.
 26. The method of claim 14 wherein saidmaximum current density is less than 12 milliamps per square centimeter.27. The method of claim 14 wherein said number of cells are in seriesand constitute a PV module.
 28. The method of claim 27 wherein saidelectrolysis system comprises a plurality of said PV modules.
 29. Themethod of claim 28 wherein each said PV module produces the optimumvoltage to produce hydrogen in a respective electrolysis circuitcomprising an anode, cathode, and electrolyte solution.
 30. Anelectrolysis system comprising at least one photovoltaic (PV) celldelivering 1.8 to 2.5 volts DC (E_(oper)), current density less than 12milliamps per cm², said PV cell having a nickel-based cathode, and ananode comprising a ruthenium oxide layer and a nickel-based layer. 31.The electrolysis system of claim 30 which comprises an electrolytecomprising KOH.
 32. The electrolysis system of claim 30 which comprisesan electrolyte comprising 5 molar KOH.
 33. The electrolysis system ofclaim 30 wherein the PV cell has an open circuit voltage of 2 to 4 voltsDC.
 34. The electrolysis system of claim 30 wherein the PV cell has anopen circuit voltage of 2.5 to 3.5 volts DC.
 35. An electrolysis systemhaving at least one photovoltaic (PV) cell supplying power toelectrolyze water to produce hydrogen, comprising: a number of PV cellsin series having a voltage sufficient to electrolyze water and satisfyelectrolysis system losses based on a maximum power point voltage ofsaid PV cell.